Give the queue head the ability to stop stealers. After a number of
spins without sucessfully acquiring the lock, the queue head sets
this, which halts stealing and will assure it is the next owner.
Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
arch/powerpc/include/asm/qspinlock_types.h | 8 +++-
arch/powerpc/lib/qspinlock.c | 53 ++++++++++++++++++++++
2 files changed, 60 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/qspinlock_types.h
b/arch/powerpc/include/asm/qspinlock_types.h
index b9a5a52fa670..1911a8a16237 100644
--- a/arch/powerpc/include/asm/qspinlock_types.h
+++ b/arch/powerpc/include/asm/qspinlock_types.h
@@ -29,7 +29,8 @@ typedef struct qspinlock {
* Bitfields in the lock word:
*
* 0: locked bit
- * 1-16: unused bits
+ * 1-15: unused bits
+ * 16: must queue bit
* 17-31: tail cpu (+1)
*/
#define _Q_SET_MASK(type) (((1U << _Q_ ## type ## _BITS) - 1)\
@@ -39,6 +40,11 @@ typedef struct qspinlock {
#define _Q_LOCKED_BITS 1
#define _Q_LOCKED_VAL (1U << _Q_LOCKED_OFFSET)
+/* 0x00010000 */
+#define _Q_MUST_Q_OFFSET 16
+#define _Q_MUST_Q_BITS 1
+#define _Q_MUST_Q_VAL (1U << _Q_MUST_Q_OFFSET)
+
/* 0xfffe0000 */
#define _Q_TAIL_CPU_OFFSET 17
#define _Q_TAIL_CPU_BITS 15
diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 6d67bc38b122..979b17ac7bd1 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -22,6 +22,7 @@ struct qnodes {
/* Tuning parameters */
static int steal_spins __read_mostly = (1<<5);
static bool maybe_stealers __read_mostly = true;
+static int head_spins __read_mostly = (1<<8);
static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
@@ -30,6 +31,11 @@ static __always_inline int get_steal_spins(void)
return steal_spins;
}
+static __always_inline int get_head_spins(void)
+{
+ return head_spins;
+}
+
static inline u32 encode_tail_cpu(int cpu)
{
return (cpu + 1) << _Q_TAIL_CPU_OFFSET;
@@ -104,6 +110,22 @@ static __always_inline u32 publish_tail_cpu(struct
qspinlock *lock, u32 tail)
return prev;
}
+static __always_inline u32 set_mustq(struct qspinlock *lock)
+{
+ u32 prev;
+
+ asm volatile(
+"1: lwarx %0,0,%1 # set_mustq \n"
+" or %0,%0,%2 \n"
+" stwcx. %0,0,%1 \n"
+" bne- 1b \n"
+ : "=&r" (prev)
+ : "r" (&lock->val), "r" (_Q_MUST_Q_VAL)
+ : "cr0", "memory");
+
+ return prev;
+}
+
static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
{
int cpu = decode_tail_cpu(val);
@@ -139,6 +161,9 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
do {
u32 val = READ_ONCE(lock->val);
+ if (val & _Q_MUST_Q_VAL)
+ break;
+
if (unlikely(!(val & _Q_LOCKED_VAL))) {
if (__queued_spin_trylock_steal(lock))
return true;
@@ -157,7 +182,9 @@ static inline void queued_spin_lock_mcs_queue(struct
qspinlock *lock)
struct qnodes *qnodesp;
struct qnode *next, *node;
u32 val, old, tail;
+ bool mustq = false;
int idx;
+ int iters = 0;
BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
@@ -209,6 +236,15 @@ static inline void queued_spin_lock_mcs_queue(struct
qspinlock *lock)
break;
cpu_relax();
+ if (!maybe_stealers)
+ continue;
+ iters++;
+
+ if (!mustq && iters >= get_head_spins()) {
+ mustq = true;
+ set_mustq(lock);
+ val |= _Q_MUST_Q_VAL;
+ }
}
/* If we're the last queued, must clean up the tail. */
@@ -293,9 +329,26 @@ static int steal_spins_get(void *data, u64 *val)
DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_spins_set,
"%llu\n");
+static int head_spins_set(void *data, u64 val)
+{
+ head_spins = val;
+
+ return 0;
+}
+
+static int head_spins_get(void *data, u64 *val)
+{
+ *val = head_spins;
+
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_set,
"%llu\n");
+
static __init int spinlock_debugfs_init(void)
{
debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL,
&fops_steal_spins);
+ debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL,
&fops_head_spins);
return 0;
}
--
2.37.2
